U.S. patent number 6,734,603 [Application Number 08/797,553] was granted by the patent office on 2004-05-11 for thin layer composite unimorph ferroelectric driver and sensor.
This patent grant is currently assigned to The United States of America as represented by the National Aeronautics and Space Administration, The United States of America as represented by the National Aeronautics and Space Administration. Invention is credited to Robert G. Bryant, Robert L. Fox, Richard F. Hellbaum, Antony Jalink, Jr., Wayne W. Rohrbach, Joycelyn O. Simpson.
United States Patent |
6,734,603 |
Hellbaum , et al. |
May 11, 2004 |
Thin layer composite unimorph ferroelectric driver and sensor
Abstract
A method for forming ferroelectric wafers is provided. A
prestress layer is placed on the desired mold. A ferroelectric
wafer is placed on top of the prestress layer. The layers are
heated and then cooled, causing the ferroelectric wafer to become
prestressed. The prestress layer may include reinforcing material
and the ferroelectric wafer may include electrodes or electrode
layers may be placed on either side of the ferroelectric layer.
Wafers produced using this method have greatly improved output
motion.
Inventors: |
Hellbaum; Richard F. (Hampton,
VA), Bryant; Robert G. (Poquoson, VA), Fox; Robert L.
(Hayes, VA), Jalink, Jr.; Antony (Newport News, VA),
Rohrbach; Wayne W. (Yorktown, VA), Simpson; Joycelyn O.
(Hampton, VA) |
Assignee: |
The United States of America as
represented by the National Aeronautics and Space
Administration (Washington, DC)
|
Family
ID: |
23650582 |
Appl.
No.: |
08/797,553 |
Filed: |
January 24, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
416598 |
Apr 4, 1995 |
5632841 |
|
|
|
Current U.S.
Class: |
310/330;
310/332 |
Current CPC
Class: |
H01L
41/0926 (20130101); H01L 41/313 (20130101); B32B
37/144 (20130101); H01L 41/098 (20130101); Y10T
29/42 (20150115) |
Current International
Class: |
B32B
37/14 (20060101); H01L 41/09 (20060101); H01L
41/24 (20060101); H01L 041/08 () |
Field of
Search: |
;156/160,163,165,245
;29/25.35 ;310/330,331,332,340,345,367,369,370,371,311,328
;264/229 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Budd; Mark
Attorney, Agent or Firm: Edwards; Robin W. Hammerle; Kurt G.
Blackburn; Linda B.
Government Interests
ORIGIN OF THE INVENTION
The invention described herein was made by employees of the United
States Government and may be used by and for the Government for
governmental purposes without the payment of any royalties thereon
or therefor.
Parent Case Text
This application is a continuing application of commonly-owned
patent application Ser. No. 08/416,598, filed Apr. 4, 1995, now
U.S. Pat. No. 5,632,841.
Claims
What is claimed is:
1. An electroactive device providing large mechanical output
displacements, comprising: a layered structure having a
prestressing layer having a convex surface, and a piezoelectric
layer having a concave surface, the convex surface of the
prestressing layer being bonded onto the concave surface of the
piezoelectric layer such that the prestressing layer is in tension
and imparts a prestress on the piezoelectric layer such that the
piezoelectric layer is in compression, wherein the prestressing
layer and the piezoelectric layer are distinct from one
another.
2. The device of claim 1, wherein the prestressing layer includes
reinforcing material.
3. The device of claim 1, wherein the piezoelectric layer includes
surface electrodes.
4. The device of claim 1, further comprising: an electrode layer
placed between the prestressing layer and the piezoelectric layer;
and an electrode layer placed on top of the piezoelectric
layer.
5. The device of claim 1, wherein the prestressing layer is an
adhesive.
6. The device of claim 1, where the piezoelectric layer is a
ferroelectric material.
7. The device of claim 1, wherein the piezoelectric layer is a
piezorestrictive material.
8. The device of claim 5, wherein the adhesive is a polyimide.
Description
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates generally to ferroelectric devices,
and more particularly to ferroelectric devices providing large
mechanical output displacements.
2. Discussion of the Related Art
Prior art methods include `Rainbow` piezoelectric actuators and
sensors, more conventional piezoelectric actuators and sensors, and
electro-magnetic actuators.
Conventional piezoelectric actuators exhibit limited mechanical
displacements. The output of conventional piezoelectric devices is
limited by the material's basically low piezoelectric displacement
constant. Thus conventional devices of reasonable thickness (i.e.
on the order of a few millimeters) offer only micrometer-sized
mechanical output motion. `Rainbow` actuators, `Moonies`,
unimorphic, and bimorphic piezoelectric actuators exhibit greater
mechanical output motion. However even the thinnest ceramic wafers,
which exhibit the maximum observed output motion, provide a
displacement limited to approximately 1 mm of motion in the
z-direction for a device that is 3-4 cm long. Additionally 1/4 mm
thick ceramic devices are extremely brittle and fragile so that
they are prone to breakage and require special handling. Previous
methods of forming `Rainbow` actuators include an additional
chemical reduction process which releases lead vapors from the
wafer into the atmosphere.
It is accordingly an object of the present invention to provide a
ferroelectric actuator with improved mechanical displacement.
It is another object of the present invention to provide a
ferroelectric actuator with improved durability.
It is another object of the present invention to provide a
ferroelectric actuator with improved machinability.
It is another object of the present invention to provide a method
for producing a ferroelectric actuator which is more
environmentally safe than previous methods.
It is yet another object of the present invention to accomplish the
foregoing objects in a simple manner.
Additional objects and advantages of the present invention are
apparent from the drawings and specification which follow.
SUMMARY OF THE INVENTION
According to the present invention, the foregoing and additional
objects are obtained by providing a method for producing
ferroelectric devices. First, a mold is selected for the device. A
prestress layer is placed on the mold and a ferroelectric layer is
placed on top of the prestress layer. These layers are bonded
together by heating and then cooling the assembled device. The
prestress layer may be an adhesive and may include reinforcing
material. The ferroelectric layer may be a piezoelectric material,
a piezostrictive material or a composite. The ferroelectric layer
includes surface electrodes which may be applied by including an
electrode layer on either side of the ferroelectric layer prior to
heating the assembly.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of the preferred embodiment prior to
bonding the layers;
FIG. 2 is a cross sectional view of the preferred embodiment after
cooling of the layers;
FIG. 3 is a cross sectional view of an alternate embodiment of the
present invention;
FIG. 4 is a cross sectional view of an alternate embodiment of the
present invention;
FIG. 5 is a cross sectional view showing the manufacturing process
of the present invention;
FIG. 6 is perspective view showing an alternate embodiment of the
present invention;
FIG. 7 is a top view showing a plurality of prestressed wafers
connected to form a larger wafer; and
FIG. 8 is a side view showing three of the prestressed wafers in a
stacked configuration.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows a piezoelectric device 10 according to the present
invention prior to being processed. The device includes four
layers, a piezoelectric layer 12, a prestressing layer 14 and two
electrode layers 18a and 18b. The piezoelectric layer 12 can be
made from a disk of piezoelectric material commercially available
from Aura Ceramics (C3900 material) or Morgan Matrox.
Alternatively, this layer can be made from piezoelectric material
that was first ground to a fine powder and subsequently
consolidated into a layer by compression bonding the powder with an
adhesive such as a polyimide, as shown in "Tough, Soluble,
Aromatic, Thermoplastic Copolyimides", Ser. No. 08/359,752, filed
Dec. 16, 1994. Note that in the latter approach, the adhesive
binder makes up a very small fraction, typically 5 percent by
weight, of the finished piezoelectric layer 12. This latter
approach is attractive since the required bonding operation can
simply be performed simultaneously with other needed bonding
operations discussed in the next paragraph. In addition to
piezoelectric materials, other ferroelectric materials, including
piezostrictive materials may be used to form this layer.
The prestressing layer 14 can be made of a mechanically strong
adhesive such as a polyimide. Thermoplastics, thermosets and braze
alloys may also be used for this layer 14. Additionally, multiple
prestress layers 14 may be used to provide increased prestress. The
adhesive is wet-coated or a thin film is melted onto one surface of
the piezoelectric layer 12 and then bonded to it at an elevated
temperature which is dependent on the adhesive being used and
allows the material to undergo cure, drying, and/or melt flow. The
layer of adhesive thus formed is typically twelve microns thick,
but can range in thickness from a few microns to several mm.
Bonding of the layers occurs at a high temperature, which depends
upon the adhesive but is typically 200-350.degree. C., so that when
the two-layer composite matrix cools to room temperature, the
differential thermal compression rates of the layers automatically
impart the desired mechanical prestress into the layers, as shown
in FIG. 2. If desired, the prestressing layer 14 of adhesive can be
reinforced primarily to allow augmenting the level of prestress,
but also for mechanical toughness and decreased hysteresis. To
accomplish this, a thin layer of reinforcing material 16 is fused
or bonded onto (FIG. 3), or into (FIG. 4), the prestressing layer
14. Examples of reinforcing materials include, but are not limited
to, plastics, ceramics, metals and combinations of these materials
such as aluminum sheet stock and carbon fibers. Bonding of the
reinforcing material 16 can occur simultaneously with the bonding
of the piezoelectric to the prestressing layer.
The adhesive layer allows the thin ceramic wafer to be cut to shape
without chipping or fracturing using conventional cutting tools
like scissors and pattern cutters allowing tailor-made shapes
rather than molded shapes. This method enables techniques to be
used which allow the pattern building of 3-dimensional objects from
the consolidation of the 2-dimensional flat ceramic shapes. These
layers can also offer additional environmental protection which
allows these devices to be used in harsh conditions. If the
adhesive layer used is a good dielectric, the chances of internal
and external arcing due to the applied voltage are reduced.
In one embodiment, the piezoelectric device 10 according to the
present invention contains two electrodes 18a and 18b. The
electrodes 18a and 18b can be of the more conventional
vacuum-deposited metal type, and can be applied onto the
piezoelectric layer 12 prior to application of the prestressing
layer 14. Alternatively, the electrodes can be a graphite or
metal-plus-adhesive mixture such as silver epoxy, which is an
excellent conductor. This alternate technique has the advantage
that the conductive adhesive mixture can be coated onto the
piezoelectric layer 12 and subsequently bonded to the piezoelectric
layer 12, simultaneous with the bonding of the prestressing 14 and
piezoelectric layers 12. Multiple or patterned electrodes may also
be used if necessary for the desired application.
The above teachings, may be combined to simplify the manufacture of
piezoelectric devices. Complete devices can be produced by
assembling separate layers of different materials, such as the
appropriate mixtures of adhesively coated powdered piezoelectric
material plus adhesive for the piezoelectric layer, conductive
adhesive for the electrodes, and the adhesive by itself or as
reinforcement for the prestressing layer, followed by a single
high-temperature bonding operation as described in "Tough, Soluble,
Aromatic, Thermoplastic Copolyimides", Ser. No. 08/359,752, filed
Dec. 16, 1994.
Provisions should be made during the manufacturing process to
ensure that the finished piezoelectric device has its prestressing
layer in tension which places the piezoelectric material in the
desired compression. The amount of prestress in the piezoelectric
material can be tailored during manufacture in order to maximize
the output motion and efficiency of the final device. The material
layers may be formed on a curve-shaped mold.
A description typical of fabricating a prestressed device 10 by
hand is provided here and shown in FIG. 5. A molding surface 20 is
selected for the amount of curvature needed to provide the desired
prestress. The prestress reinforcing layer 16 of aluminum foil is
then placed on top of the molding surface 20. Next the adhesive
prestress layer 14 made from a polyimide as described in "Tough,
Soluble, Aromatic, Thermoplastic Copolyimides", Ser. No.
08/359,752, filed Dec. 16, 1994 is placed on top of the reinforcing
layer 16. The electrode layer 18b of silver is vacuum deposited on
the lower surface of the piezoelectric wafer 12 (this step is
unnecessary if pre-electroded piezoelectric wafers are being used).
The piezoelectric wafer 12 is placed on top of the adhesive
prestress layer 14. Finally, an electrode layer 18a of silver is
vacuum deposited on the upper surface of piezoelectric wafer 12, if
necessary. A sheet of high temperature material 22, such as
Kapton.RTM. (DuPont), is placed over the stack and is sealed using
high temperature bagging tape 24 to produce a vacuum bag. The
assembly is placed into an oven and the air in the Kapton.RTM. bag
22 is evacuated through vacuum port 26. The oven is heated to
300.degree. C. to melt the adhesive and bond the assembly. Upon
cooling, the assembly undergoes further prestressing, and the
resulting piezoelectric device is removed from the vacuum bag and
mold.
Although the ferroelectric wafers are typically poled as received
from the vendor, they must be repoled following thermal treatment
in the prestress process. The poling is done at an elevated
temperature with a DC voltage sufficient to induce dipole
orientation. After poling, the wafer is cooled to room temperature
in the presence of the electric field to preserve the induced
orientation. The DC field strength employed in the polarization is
selected to obtain optimum polarization without exceeding the field
at which dielectric breakdown occurs in the material at a given
poling temperature.
The amount and type of input voltage per unit of deflection,
motion, force and output voltage, current, or power conversion can
be adjusted by varying the thickness and/or number of layers of the
piezoelectric, the number of layers and/or thickness of the
prestress layer, the prestress material, the piezoelectric
composition and the curvature and shape of the molding surface. By
varying the curvature of the mold, the prestress imparted to the
finished piezoelectric device is varied. By varying the thickness
or number of prestress material layers or by varying the material
used, the output motion and mechanical force can also be varied.
During processing, the piezoelectric and prestressing layers move
with respect to each other and upon cooling bond together with
additional prestress. This method of making devices has shown
substantial increase of output motion of otherwise identical
piezoelectric devices.
A cylindrical bender mode may be emphasized by prestressing in only
one direction which can be done by bending the layers over a
cylindrical molding surface during the heating cycle. On cooling,
the prestressing layer 14, being under the piezoelectric layer 12
has a tighter radius of curvature and prestresses more in one
direction thus forming the bender. These cylindrical mode benders
are typically shapes other than circular as shown in FIG. 6.
A number of individual, polygon-shaped piezoelectric devices 28 can
be grouped into a large-area mosaic by joining their appropriate
edges as shown in FIG. 7. One useful method for edge attachment is
the use of a single reinforcing layer that covers the entire
mosaic.
Certain applications may require more mechanical output force than
can be provided by a single device 10. Two or more devices 10 can
then be used in an efficient tandem arrangement by uniting their
dome-like shapes in a `stacked-spoons` configuration. FIG. 8 shows
three devices in this stacked configuration. To allow unimpeded
bending of the individual devices during operation the devices can
be bonded to each other using a compliant layer 30, such as an
elastomer, i.e. silicone rubber, which allows one layer to move
relative to the other. In such an actuator stack, the individual
devices 10 remain electrically isolated from each other; one or
more of the devices 10 can act as a motion feedback sensor.
When made using a matrix composite fabrication method shown in
"Tough, Soluble, Aromatic, Thermoplastic Copolyimides", Ser. No.
08/359,752, filed Dec. 16, 1994, large flexible sheets may be
produced for use in low-frequency actuator applications (i.e. noise
canceling devices or loud speakers). This can be accomplished by
using large flat molds for consolidation or a continuous roll
process. Molded parts can be bonded together by heating them to
soften and/or cure the binder adhesive while pressing them
together.
Ferroelectric devices made from the present method can be used in
pumps, valves, brakes, motors, sensors, actuators, optics, acoustic
transducers, and active structures.
Many improvements, modifications, and additions will be apparent to
the skilled artisan without departing from the spirit and scope of
the present invention as described herein and defined in the
following claims.
* * * * *